[Bug c/45819] New: [4.5 Regression] unexpected unaligned access to volatile int

[Bug c/45819] New: [4.5 Regression] unexpected unaligned access to volatile int

[anemo at mba dot ocn.ne.jp]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=45819

           Summary: [4.5 Regression] unexpected unaligned access to
                    volatile int
           Product: gcc
           Version: 4.5.1
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c
        AssignedTo: unassigned@gcc.gnu.org
        ReportedBy: anemo@mba.ocn.ne.jp


An unexpected unaligned access instructions for volatile int are
generated on gcc 4.5 or 4.6 with this test case.

struct st {
    int ptr;
} __attribute__ ((packed));

int foo(struct st *st)
{
    int v = *(volatile int *)&st->ptr;
    return v;
}

This test case is derived from something like this in ARM linux kernel.

struct st {
    unsigned int ptr;
} __attribute__ ((packed));
unsigned int foo(struct st *st)
{
    return readl(&st->ptr);
}

mipsel-linux-gcc-4.5.1 -O2 foo.c -S output:
    lwl    $2,3($4)
    nop
    lwr    $2,0($4)
    j    $31
    nop
arm-linux-gnueabi-gcc-4.5.1 -O2 foo.c -S output:
    ldrb    r3, [r0, #0]    @ zero_extendqisi2
    ldrb    r1, [r0, #1]    @ zero_extendqisi2
    ldrb    r2, [r0, #2]    @ zero_extendqisi2
    orr    r3, r3, r1, asl #8
    ldrb    r0, [r0, #3]    @ zero_extendqisi2
    orr    r3, r3, r2, asl #16
    orr    r0, r3, r0, asl #24
    bx    lr

It seems the cast is ignored and unaligned access is assumed.
gcc 4.4.4 works fine.
mipsel-linux-gcc-4.4.4 -O2 foo.c -S output:
    lw    $2,0($4)
    j    $31
    nop
arm-linux-gnueabi-gcc-4.4.4 -O2 foo.c -S output:
    ldr    r0, [r0, #0]
    bx    lr

The similar problem was found as PR 45704.

git-bisect tell me same commit
0d9f1189f3df5ce5c0efc3ecadc7c0a4f75b202d is the first bad commit.

Like PR 45704, reverting this change fixes this problme too.

-  STRIP_USELESS_TYPE_CONVERSION (sub);
+  STRIP_NOPS (sub);

But the final fix for PR 45704 does not fix this problem.

mipsel-linux-gcc (4.6.0 20100927):
    lbu    $5,0($4)
    lbu    $6,1($4)
    lbu    $3,2($4)
    andi    $6,$6,0x00ff
    lbu    $2,3($4)
    andi    $5,$5,0x00ff
    sll    $6,$6,8
    andi    $3,$3,0x00ff
    or    $4,$6,$5
    sll    $3,$3,16
    or    $3,$3,$4
    sll    $2,$2,24
    j    $31
    or    $2,$2,$3
arm-linux-gnueabi-gcc (4.6.0 20100927):
    ldrb    r3, [r0, #0]    @ zero_extendqisi2
    ldrb    r1, [r0, #1]    @ zero_extendqisi2
    ldrb    r2, [r0, #2]    @ zero_extendqisi2
    orr    r3, r3, r1, asl #8
    ldrb    r0, [r0, #3]    @ zero_extendqisi2
    orr    r3, r3, r2, asl #16
    orr    r0, r3, r0, asl #24
    bx    lr

[Bug c/45819] [4.5 Regression] unexpected unaligned access to volatile int

[rguenth at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=45819

--- Comment #1 from Richard Guenther <rguenth at gcc dot gnu.org> 2010-09-28 16:04:38 UTC ---
As a matter of clean implementation I suggest to do

struct st {
    int ptr;
} __attribute__ ((packed,aligned(__alignof__(int))));

(why use packed if the int is always aligned?)

[Bug c/45819] [4.5 Regression] unexpected unaligned access to volatile int

[anemo at mba dot ocn.ne.jp]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=45819

--- Comment #2 from Atsushi Nemoto <anemo at mba dot ocn.ne.jp> 2010-09-28 16:26:17 UTC ---
(In reply to comment #1)
> (why use packed if the int is always aligned?)

The original problem was found with this structure in linux ehci_def.h:
struct ehci_caps {
    u32        hc_capbase;
    u32        hcs_params;     /* HCSPARAMS - offset 0x4 */
    u32        hcc_params;      /* HCCPARAMS - offset 0x8 */
    u8        portroute [8];     /* nibbles for routing - offset 0xC */
} __attribute__ ((packed));

In this case maybe the "packed" is not needed, but there will be other cases
which require "packed" attribute on struct for memory-mapped I/O.

[Bug middle-end/45819] [4.5 Regression] unexpected unaligned access to volatile int

[anemo at mba dot ocn.ne.jp]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=45819

--- Comment #3 from Atsushi Nemoto <anemo at mba dot ocn.ne.jp> 2010-09-30 02:06:18 UTC ---
(In reply to comment #1)
> As a matter of clean implementation I suggest to do
> 
> struct st {
>     int ptr;
> } __attribute__ ((packed,aligned(__alignof__(int))));

I confirmed this fixes the problem.
But fixing all packed structure in linux kernel like this would be so hard, I
think.

[Bug middle-end/45819] [4.5 Regression] unexpected unaligned access to volatile int

[rguenth at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=45819

Richard Guenther <rguenth at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
   Target Milestone|---                         |4.5.2

[Bug middle-end/45819] [4.5 Regression] unexpected unaligned access to volatile int

[rguenth at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=45819

Richard Guenther <rguenth at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
           Priority|P3                          |P2

--- Comment #4 from Richard Guenther <rguenth at gcc dot gnu.org> 2010-11-12 13:57:45 UTC ---
Can someone check if 4.6 is really not affected?

[Bug middle-end/45819] [4.5 Regression] unexpected unaligned access to volatile int

[anemo at mba dot ocn.ne.jp]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=45819

--- Comment #5 from Atsushi Nemoto <anemo at mba dot ocn.ne.jp> 2010-11-24 14:34:00 UTC ---
(In reply to comment #4)
> Can someone check if 4.6 is really not affected?

arm-linux-gnueabi-gcc-4.6.0-20101124 works fine (generates ldr instruction),
but mipsel-linux-gcc-4.6.0-20101124 still generates lbu (load-byte-unsigned)
instructions.

[Bug middle-end/45819] [4.5 Regression] unexpected unaligned access to volatile int

[rguenth at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=45819

Richard Guenther <rguenth at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
   Target Milestone|4.5.2                       |4.5.3

--- Comment #6 from Richard Guenther <rguenth at gcc dot gnu.org> 2010-12-16 13:02:50 UTC ---
GCC 4.5.2 is being released, adjusting target milestone.

[Bug middle-end/45819] [4.5 Regression] unexpected unaligned access to volatile int

[anemo at mba dot ocn.ne.jp]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=45819

--- Comment #7 from Atsushi Nemoto <anemo at mba dot ocn.ne.jp> 2011-02-22 13:04:10 UTC ---
(In reply to comment #5)
> arm-linux-gnueabi-gcc-4.6.0-20101124 works fine (generates ldr instruction),

It seems that was a side-effect of -fstrict-volatile-bitfields which was
enabled
by default on ARM EABI.
With -fno-strict-volatile-bitfields, arm-linux-gnueabi-gcc-4.6.0-20110222
generates
four ldrb instructions.

[Bug middle-end/45819] [4.5 Regression] unexpected unaligned access to volatile int

[rguenth at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=45819

Richard Guenther <rguenth at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
   Target Milestone|4.5.3                       |4.5.4

--- Comment #8 from Richard Guenther <rguenth at gcc dot gnu.org> 2011-04-28 14:51:01 UTC ---
GCC 4.5.3 is being released, adjusting target milestone.

[Bug middle-end/45819] [4.5 Regression] unexpected unaligned access to volatile int

[raj.khem at gmail dot com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=45819

Khem Raj <raj.khem at gmail dot com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |raj.khem at gmail dot com

--- Comment #9 from Khem Raj <raj.khem at gmail dot com> 2011-06-09 06:47:01 UTC ---
here is another testcase which generates loadbytes with gcc 4.6 but works as
expected with gcc 4.5 on arm. gcc are latest from respective branches. and it
fails irrespective of having -fstrict-volatile-bitfields or not.

struct ehci_regs {                                                              
char x;                                                                         
unsigned int port_status[0];                                                    
} __attribute__ ((packed));                                                     
//} __attribute__ ((packed,aligned(__alignof__(int))));                         

struct ehci_hcd{                                                                
struct ehci_regs *regs;                                                         
};                                                                              

int ehci_hub_control (                                                          
 struct ehci_hcd *ehci,                                                         
 int wIndex                                                                     
) {                                                                             
 unsigned int *status_reg = &ehci->regs->port_status[wIndex];                   
 return *(volatile unsigned int *)status_reg;                                   
}

[Bug middle-end/45819] [4.5 Regression] unexpected unaligned access to volatile int

[rguenth at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=45819

Richard Guenther <rguenth at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |ASSIGNED
   Last reconfirmed|                            |2011.07.22 09:49:13
         AssignedTo|unassigned at gcc dot       |rguenth at gcc dot gnu.org
                   |gnu.org                     |
     Ever Confirmed|0                           |1

--- Comment #10 from Richard Guenther <rguenth at gcc dot gnu.org> 2011-07-22 09:49:13 UTC ---
Testing a fix for the volatile issue in comment #9.

I can't reproduce anything wrong with the testcase from the initial comment,
that looks like a target / expander issue.

The issues probably should have had different bugs instead of lumping them
together here.

[Bug middle-end/45819] [4.5 Regression] unexpected unaligned access to volatile int

[rguenth at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=45819

--- Comment #11 from Richard Guenther <rguenth at gcc dot gnu.org> 2011-07-22 11:55:33 UTC ---
Author: rguenth
Date: Fri Jul 22 11:55:30 2011
New Revision: 176623

URL: http://gcc.gnu.org/viewcvs?root=gcc&view=rev&rev=176623
Log:
2011-07-22  Richard Guenther  <rguenther@suse.de>

    PR tree-optimization/45819
    * tree-ssa-forwprop.c (forward_propagate_addr_expr_1): Properly
    preserve volatile and notrap flags.

    * gcc.dg/pr45819.c: New testcase.

Added:
    trunk/gcc/testsuite/gcc.dg/pr45819.c
Modified:
    trunk/gcc/ChangeLog
    trunk/gcc/testsuite/ChangeLog
    trunk/gcc/tree-ssa-forwprop.c

[Bug middle-end/45819] [4.5 Regression] unexpected unaligned access to volatile int

[rguenth at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=45819

--- Comment #12 from Richard Guenther <rguenth at gcc dot gnu.org> 2011-07-22 12:19:25 UTC ---
Author: rguenth
Date: Fri Jul 22 12:19:21 2011
New Revision: 176624

URL: http://gcc.gnu.org/viewcvs?root=gcc&view=rev&rev=176624
Log:
2011-07-22  Richard Guenther  <rguenther@suse.de>

    PR tree-optimization/45819
    * tree-ssa-forwprop.c (forward_propagate_addr_expr_1): Properly
    preserve volatile and notrap flags.

    * gcc.dg/pr45819.c: New testcase.

Added:
    branches/gcc-4_6-branch/gcc/testsuite/gcc.dg/pr45819.c
Modified:
    branches/gcc-4_6-branch/gcc/ChangeLog
    branches/gcc-4_6-branch/gcc/testsuite/ChangeLog
    branches/gcc-4_6-branch/gcc/tree-ssa-forwprop.c

[Bug middle-end/45819] [4.5 Regression] unexpected unaligned access to volatile int

[rguenth at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=45819

Richard Guenther <rguenth at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|ASSIGNED                    |NEW
         AssignedTo|rguenth at gcc dot gnu.org  |unassigned at gcc dot
                   |                            |gnu.org

--- Comment #13 from Richard Guenther <rguenth at gcc dot gnu.org> 2011-07-22 12:20:30 UTC ---
Comment #9 should be fixed now.  That leaves the initial report which I
can't reproduce - thus, not mine anymore.

Please someone verify comment #9 on arm.

[Bug middle-end/45819] [4.5 Regression] unexpected unaligned access to volatile int

[rguenth at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=45819

--- Comment #14 from Richard Guenther <rguenth at gcc dot gnu.org> 2011-07-22 12:21:30 UTC ---
Oh, the initial testcase was invalid anyway.

[Bug middle-end/45819] [4.5 Regression] unexpected unaligned access to volatile int

[rguenth at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=45819

Richard Guenther <rguenth at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|NEW                         |RESOLVED
         Resolution|                            |INVALID

--- Comment #15 from Richard Guenther <rguenth at gcc dot gnu.org> 2011-07-22 12:25:01 UTC ---
struct ehci_regs {                                                              
char x;                                                                         
unsigned int port_status[0];                                                    
} __attribute__ ((packed));                                                     
//} __attribute__ ((packed,aligned(__alignof__(int))));                         

struct ehci_hcd{                                                                
struct ehci_regs *regs;                                                         
};                                                                              

int ehci_hub_control (                                                          
 struct ehci_hcd *ehci,                                                         
 int wIndex                                                                     
) {                                                                             
 unsigned int *status_reg = &ehci->regs->port_status[wIndex];                   
 return *(volatile unsigned int *)status_reg;                                   
}

this one is invalid as well, with or without the aligned attribute.

ehci->regs->port_status[wIndex] _is_ unaligned.  I don't think there
is currently a way to tell GCC that the struct layout of ehci_regs is
packed but port_status is properly aligned to the natural alignment of int.